The invention relates to a process for preparing pearl luster pigments based on metal oxide coated mica flakes wherein, to coat with titanium dioxide in the rutile form, not only TiO.sub.2 but also SnO.sub.2 is precipitated onto the mica flakes in aqueous suspension and the pigment is then washed, dried and calcined.
Processes for preparing rutile-containing mica pigments are known not only from German Patent Specifications Nos. 2,214,545 and 2,522,572 (equivalent to U.S. Pat. No. 4,086,100) but also from U.S. Pat. No. 4,038,099.
In the process of DE Patent No. 2,522,572, first a very thin titanium dioxide layer is precipitated onto mica flakes followed alternatingly by at least one SnO.sub.2 and TiO.sub.2 layer, thicker layers being prepared by alternatingly precipitating a plurality of SnO.sub.2 and TiO.sub.2 layers. These SnO.sub.2 and TiO.sub.2 layers are in each case prepared by slowly adding to the mica suspension, together with a base for keeping the pH constant, the tin salt solution with or without an oxidizing agent in the one case and the titanium salt solution in the other. In this way the precipitation can be very simply continued until the metal oxides are present in the desired layer thickness and consequently the pigment has the desired interference color. Customary washing, drying and calcining produces very brilliant pigments. However, a disadvantage is the very complicated process, in particular in the case of relatively thick layers. In addition, it requires relatively high tin dioxide contents, which in general range from about 5 to 7% by weight, based on the completed pigment.
It is true that the process of DE Patent 2,214,545 and U.S. Pat. No. 4,038,099 can be carried out with smaller amounts of tin, but other serious disadvantages remain. In this process, the mica suspension has added to it the solution of a tin salt and then a strong acid, and a portion of the tin precipitates as water-containing tin oxide on the mica flakes. Thereafter a titanyl sulfate solution is added, and the suspension is heated to a boil; as a result of hydrolysis and precipitation, titanium dioxide deposits on the mica flakes and, in the course of this process, the remaining tin is incorporated as a tin oxide in the titanium dioxide layer as it forms.
In this hydrolysis process, the thickness of the metal oxide layer and hence the interference color of the pigment is determined from the start by the amount of metal salt added. Corrections in the course of the coating process and in particular accurate end point determination as in the process of DE Patent No. 2,522,572 are not possible.
However, it has been found, surprisingly, that it is not even possible to combine the tin dioxide precipitation as described in DE Patent No. 2,214,545 with the titanium dioxide precipitation as described in DE Patent No. 2,522,572. Evidently, the tin salt remaining in solution interferes with the titanium dioxide precipitation to such an extent as to produce only qualitatively very poor pigments whose rutile content is a non-reproducible 100%.